1. Field of the Invention
The present invention relates to a wafer mapping apparatus for detecting presence/absence of wafers on shelves provided in the interior of a clean box for storing semiconductor wafers or the like in a manufacturing process for a semiconductor device, electronic parts and related goods, and optical disks etc.
2. Related Background Art
Recently, in the wafer processing process in the manufacturing process of semiconductor devices or the like that requires a high degree of cleanness, there has been adopted a method in which a highly clean environment is not established in the whole of the room related to the processing. In that method, a mini-environment space that is kept in a highly clean condition is provided in each of wafer processing apparatuses used in the wafer manufacturing process. This method is intended to keep only small spaces, that is, the spaces within wafer processing apparatuses and wafer storing containers (which will be referred to as a pod, hereinafter) for storing wafers during transferring of wafer between the wafer processing apparatuses, instead of keeping a large space such as the room related to the processing. With this method, it is possible to save initial investment and maintenance costs that would be required in the case that the whole room related to the processing of wafers is kept in a highly clean condition, while realizing the effects same as those attained by establishing a highly clean environment within the whole of the room related to the wafer processing to realize an effective manufacturing process.
In the interior of the pod, there is provided a rack having shelves on which wafers are to be placed. Wafers are stored in the rack in such a way that one wafer is allotted to one of the shelves. The wafers received in the rack are transferred between the wafer processing apparatuses with the transferred pod. However, in the course of the processing process performed by the wafer processing apparatuses, wafers that are out of predetermined standards are sometimes generated. Such wafers are removed from the shelve in the pod. Therefore, even if the all of the shelves of the rack are filled with wafers at the initial stage of the manufacturing process, the number of the shelves on which no wafer is placed will increase due to the removals of wafers as the processing process by the wafer processing apparatuses advances.
Since the wafer processing apparatuses performs processing on wafers automatically, each wafer processing apparatus is generally provided with a wafer transferring robot (which will be simply referred to as a transfer robot hereinafter). The transfer robot accesses a shelf of the pod to transfer or bring a wafer for the wafer processing process. If the transfer robot accesses a shelf in which a wafer is not present for the purpose of transferring a wafer in spite that the wafer to be processed is not present in that shelf, there occurs a useless movement process, namely, the robot uselessly accesses the shelf and returns to the original position. As the number of times of useless movement process increases, the overall wafer processing amount decreases. In view of this, it is necessary to detect the presence/absence of the wafer in each shelf of the rack in the pod in each of the wafer processing apparatuses to determine in which shelves in the rack of the pod wafers are stored and in which shelves wafers are not stored (which determination may be called wafer mapping).
For example, in Japanese Patent Application No. 2001-158458 a semiconductor wafer processing apparatus having a wafer mapping function is proposed. The apparatus disclosed in Japanese Patent Application No. 2001-158458 performs wafer mapping using a transmissive sensor for wafer detection including a paired emitter and detector and a dog having regularly formed notches or indentations and a transmissive sensor for the dog arranged in a manner embracing the portion of the dog in which notches are formed. This Japanese Patent Application teaches arranging the emitter and the detector of the transmissive wafer detection sensor opposed to each other with a certain distance therebetween and moving the emitter and the detector in the direction perpendicular to the shelves on which wafers are placed to detect presence/absence of the wafers.
Specifically, when a wafer blocks light from the emitter and the detector does not detect light from the emitter, a non-transmission signal is generated, so that it is determined that a wafer is present on the shelf. On the other hand, when a wafer is not present on the shelf and the detector receives light from the emitter, a transmission signal is generated, so that it is determined that a wafer is present on the shelf. The above-mentioned structure is arranged in such a way that the timing of a signal generated upon detection of each notch as the transmissive sensor for the dog moves along the notches of the dog is just synchronized with the timing at which the emitter and the detector of the transmissive wafer detection sensor pass by each shelf on which a wafer is to be placed. Thus, the detector of the transmissive wafer detection sensor can detect presence/absence of the wafer without fail at the time when a wafer should present between the emitter and detector.
So far, any relevant document that constitutes a prior art has not been discovered.
(1) However, the above-described technology is a mapping technology for detecting only presence/absence of a wafer. Therefore, wafer mapping can be performed with that technology in the case that wafers are placed on the shelves of the rack in the pod on a one-by-one basis (i.e. one wafer on one shelf), but in the case that multiple wafers are placed on a shelf of the rack in the pod, it is not possible to detect the number of the wafers correctly. However, if a plurality of wafers are placed on a shelf (or shelves) of the rack of the pod, a trouble will arise in subsequent processing processes. Therefore, it is required that such a shelf (or shelves) can also be detected in the wafer mapping process performed in the wafer processing apparatus.
(2) Furthermore, in some wafer detection apparatus, driving means having a not so high stability in the speed such as an air-operated cylinder is used for moving the sensor, in order to make the structure simple. Especially, in the case of the air-operated cylinder, the variation in the speed with time is large in the early stage (or period) just after the operation of the cylinder is started or the stage (or period) in which the operation of the cylinder is stopped. In addition, even in the substantially constant speed period other than the above-mentioned periods, the variation in the speed is relatively large. Therefore, there is a problem that errors become large to spoil the accurate wafer detection when the sensor is moved by such driving means upon detection.
According to the present invention that solve the above-described problem, there is provided a wafer processing apparatus adapted to detect a wafer on each shelf of a rack having shelves on which wafers can be placed provided in a pod, comprising:
moving means that can be moved along the shelves of the rack by driving means;
a first transmissive sensor movable along the shelves of the rack by the moving means and including a first emitter and a first detector that are disposed in such a way as to be opposed to each other, the first emitter and the first detector being arranged in such a way that when the first transmissive sensor is moved along the shelves of the rack, in the case that a wafer is present on a shelf of the rack, light emitted from the first emitter toward the first detector is blocked by the wafer, and in the case that a wafer is not present on a shelf, light emitted from the first emitter is allowed to pass to the first detector;
a second transmissive sensor including a second emitter and a second detector opposed to the second emitter, the second transmissive sensor being movable along the shelves of the rack with the moving means;
a dog disposed between the second emitter and the second detector and having index means that can pass or block light emitted from the second emitter toward the second detector when the second transmissive sensor is moved along the shelves of the rack; and
a computing means for performing determination of the number of the wafer(s) placed on a shelf of the rack by comparing a wafer thickness obtained by calculating a ratio of duration time of a first signal from the first transmissive sensor corresponding to the wafer(s) and duration time of a second signal from the second sensor corresponding to the index means and a threshold value that has been set in advance in accordance with the wafer thickness and the number of wafers.
With this apparatus, it is possible to perform accurate wafer detection, even in the case that the scanning speed of a transmissive sensor is varied, without a need for complicated system. In addition, the apparatus can detect multiple wafers.